/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
struct ListNode
{
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};
#include <stack>
using namespace std;
class Solution
{
public:
    bool isPalindrome(ListNode *head)
    {
        if (head == nullptr)
        {
            return true;
        }

        // 找到前半部分链表的尾节点并反转后半部分链表
        ListNode *firstHalfEnd = endOfFirstHalf(head);
        ListNode *secondHalfStart = reverseList(firstHalfEnd->next);

        // 判断是否回文
        ListNode *p1 = head;
        ListNode *p2 = secondHalfStart;
        bool result = true;
        while (result && p2 != nullptr)
        {
            if (p1->val != p2->val)
            {
                result = false;
            }
            p1 = p1->next;
            p2 = p2->next;
        }

        // 还原链表并返回结果
        firstHalfEnd->next = reverseList(secondHalfStart);
        return result;
    }

    ListNode *reverseList(ListNode *head)
    {
        ListNode *prev = nullptr;
        ListNode *curr = head;
        while (curr != nullptr)
        {
            ListNode *nextTemp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

    ListNode *endOfFirstHalf(ListNode *head)
    {
        ListNode *fast = head;
        ListNode *slow = head;
        while (fast->next != nullptr && fast->next->next != nullptr)
        {
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }
};